1. Field of Invention:
The present invention relates to methods, materials and devices for the optical measurement of temperature, and more particularly, is directed towards new methods of using known luminescent materials for the optical measurement of temperature.
2. Description of the Prior Art:
Optical temperature sensors, namely sensors which convey temperature information in the form of optical signals, have been under development in recent years mainly in answer to a need in industry, medicine and scientific research to measure temperatures of objects and environments where the presence of electrical conductors is undesirable, such as, for instance, in the presence of microwaves or other oscillating fields. Often these sensors have been used in conjunction with optical fibers, whose function was to carry a probing light beam to the temperature sensor and to carry the optical signal produced by the sensor to a photodetector.
Another stimulus to the development of optical temperature sensors has been the growing need for sophisticated feedback and control devices in manufacturing, air and surfaces transportation, and environmental monitoring in large commercial and residential buildings. Optical sensors have an advantage over electrical sensors in that their signals can be transmitted with low attenuation, and without prior conversion or conditioning, over optical fibers, whose information-carrying capacity is greater and less subject to interference than that of electrical cables.
A further advantage of fiber optics temperature sensing systems is that they can be multiplexed so that a single light source and a single light detecting system can be used for measuring the outputs of many remote sensors simultaneously or quasi-simultaneously, with considerable cost savings over electrical sensing systems.
One of the earliest devices for the optical measurement of temperature used the temperature-dependent reflectivity of a liquid-crystalline material at the tip of a fiber bundle (Johnson, C. C. and Rozell, T. C., Microwave Journal, p. 55, August 1975). Other non-luminescent devices used the temperature-dependent light polarization of birefringent crystals (Cetas, T. C. and Connors, W. C., Medical Physics 5, 79 (1978)) and the temperature-dependent wavelength shift of the absorption edge of some semiconductors (Christensen, D. A., U.S. Pat. No. 4,136,566). All of these sensors have a relatively large thermal mass which make them unsuitable for sensitive radiometric measurements.
Only one optical temperature sensor, an indium phosphide semiconductor with a temperature-dependent absorption edge, has reportedly been uses for the measurement of cryogenic temperatures (NASA Technical Briefs, p. 55, August 1981). The procedure and instrument are relatively complicated, since a wavelength scan is required for each measurement in order to find the position of the absorption edge. As the method is not instantaneous, the system cannot be readily multiplexed for monitoring the outputs of a plurality of sensors.
Another optical method for temperature measurement makes use of temperature-dependent changes in the optical transmission of glass fibers doped with a rare earth ion having at least one electronic energy level close enough to the ground level to be thermally excited to a measurable extent (Baumbick, R. J. and Alexander, J., Control Engineering, March 1980, pp. 75-7). This level is optically coupled to higher energy levels with energy differences corresponding to specific optical wavelengths. An increase in temperature from T.sub.1 to T.sub.2 kelvins increases the occupancy number of lower, thermally excited level, by a factor approximately equal to exp[(E/k)(T.sub.1.sup.-1 -T.sub.2.sup.-1)], where E is the energy of said thermally excited level, relative to the ground level, and k is the Boltzmann constant. This results in a decreased transmission of light of said specific wavelengths. The method is suitable for measuring relatively high temperatures, such as those of the exhaust gases from aircraft engines.
Measurements of light transmission through optical temperature sensors are subject to error, especially in fiber optic systems, as changes in the optical signals caused by variable optical losses of fibers, connectors and/or couplers may be confused with the temperature-dependent signals. Techniques for minimizing such errors are relatively complex, and have included the use of a second light source to generate a reference light beam of a wavelength not subject to a temperature-dependent absorption by the sensor (Kyuma et. al., IEEE J. Quant. Electron., QE-18(4), 667 (1982)). Even then, a serious source of error remains, namely the intensity fluctuations of one or both light sources.
The use of luminescent sensors offers the advantage that the sensor itself is the second light source, which generates a resolvable luminescence light the intensity of which is proportional to the excitation light intensity absorbed by the sensor. By dividing the signal from the sensor luminescence by that from the excitation light beam transmitted through the sensor, one can essentially eliminate not only the effect of the fluctuations of the intensity of the excitation light, but also that from the optical losses in the fiber optic system. In addition to permitting the use of simpler measuring devices, luminescence techniques are more sensitive at low optical densities. Therefore, it is desirable to have sensitive luminescence methods for optical temperature measurement, preferably operable at wavelengths which are transmitted through fiber optic systems with low attenuation.
U.S. Pat. Nos. 3,639,765 and 4,061,578 describe methods for measuring infrared radiation by means of phosphors, including europium-doped terbium chelates, having a very low thermal mass and two non-overlapping luminescence spectral bands, the intensity ratio of which is a unique function of temperature, independent of intensity changes of the excitation light. The infrared radiation is absorbed by a thin black film whose temperature is increased according to the amount of infrared radiation absorbed. The temperature rise in the film is measured by a thin layer of the phosphor in thermal contact with said black film. In contrast to the other optical temperature sensors described above, the class of europium-doped terbium chelates can be used both as contact sensors and as sensitive radiometric sensors, and can measure contact temperatures over a wide range from near absolute zero to about 400 kelvins.
U.S. Pat. Nos. 4,075,493 and 4,215,275 describe inorganic europium-doped oxysulfides of lanthanum, gadolinium and yttrium as luminescent temperature sensors. Like the europium-doped terbium chelates mentioned above, these phosphors are characterized by emitting luminescence in at least two non-overlapping spectral bands the intensity ratio of which is a known function of temperature. These inorganic phosphors have a lower luminescence excitation efficiency and a much larger thermal mass than the europium-doped chelates, and cannot operate at cryogenic temperatures. They are, however, more stable at temperatures above 400 kelvins than the chelates.
Both the europium-doped oxysulfides and the europium-doped terbium require excitation with ultraviolet or violet light. This limits the distance over which the excitation light can be transmitted over optical fibers, as the corresponding wavelengths are more strongly attenuated than red or near infrared wavelengths.
A recently disclosed method for the optical measurement of temperature uses AC-modulated blue light to excite the luminescence of samarium (III) in a crystalline barium fluoride chloride host (McCormack, J. S. Electronics Letters 17, 630 (1981). The sensor temperature is determined from the decay time of the luminescence, said decay time, as well as the luminescence efficiency, decreasing with increasing temperature. The method has relatively low sensitivity and accuracy, and the need to use blue excitation light restricts the use of the method to relatively short transmission distances.
The current needs in the area of optical temperature sensing are: (a) new sensors for the successful implementation of new or previously disclosed methods for temperature measurement, and (b) improved methods for the accurate measurement of temperature, useful for wide temperature ranges, and implementable with optical wavelengths transmittable over long lengths of optical fibers with relatively little attenuation.
It is one object of the present invention to provide new luminescence methods and associated devices for the remote optical measurement of temperature, from the cryogenic region to over 800 kelvins. These methods are implementable, either throughout the whole or in portions of this wide range, with any solid or liquid luminescent material composed of molecules the electronic ground level of which comprises vibrational sublevels. This class of materials comprises most known luminescent materials.
It is another object of the present invention to provide new luminescent materials as sensors for the optical measurement of temperatures and spatial temperature distributions.
Other objects of the present invention will become apparent from the following description.
The invention accordingly comprises the methods, materials, apparatuses and systems, together with their steps, part, elements and interrelationships that are exemplified in the following disclosure, the scope of which will be indicated in the appended claims.